Present AC control systems utilize mechanical relays consuming reasonably extensive amounts of power to activate them. Some systems are now utilizing electronic relays requiring somewhat less power, but such relays still require in the range of possibly tens to hundreds of milliamps of current to activate. To turn the AC power on and off to a load such as a relay, gas valve, light or heater, at least three wires are required by the prior art. One wire is the "hot" power in from the AC power source to both the control system and the power switching device. The second wire is the return from the switching device through the load. The third wire is the common from the control system to the AC power source. Both the hot and the common are required in order to operate a dc power supply necessary to drive the logic of the control system which in turn drives the power switching device in series with the load. In many cases, this common or third wire is not available where the control system is chosen to be located. An example of where such situations arise is in the case of a home energy management system, wherein a power switching device is to be placed in series with a 24 volt or 120 volt thermostat mounted on the wall, which thermostat traditionally is in a simple series loop between the 24 volt power source and the load. The control system which drives the switching device must receive dc power as outlined above. If an additional wire is available for the common, no significant problem exists, but in many cases a separate wire must be run.
In addition, a pilot light or ON indicator is traditionally used. This is generally placed in some position across the control logic power supply, and, therefore, is in parallel with the load, and thus is an additional power or current load on the power supply. If the control logic requires a sample of the AC signal for timing or synchronizing purposes, this generally requires a halfwave rectifier in order to create positive polarity pulses compatible with the logic. At times these timing pulses must also be amplitude limited so as to not overdrive the control logic. In addition, some degree of added regulation is generally required so that the power supply maintains operation during both high and low AC line voltage. In the prior art, each of these functions required separate circuitry, each creating a load on the power source. All of this tends to create a relatively inefficient, moderately energy consuming product such that the energy required to turn on a power switching device may be as much as 1/10th the actual power being switched. As an example, some 24 volt AC energy management systems require two to four hundred milliamps of current to control a one to two amp contactor or relay. If the control circutry used has any timing mechanism that must remain functioning during an AC power outage, a battery back-up supply is usually employed. The life of this battery is inversely related to the power consumption of the system that is doing the timing. So, a list of negatives exists with current products: extra power supply wires; extra timing and regulator circuits; extra current for on/off indicators; and relatively poor efficiency in terms of power input verses power control, resulting in very limited battery back-up operating time. In addition, a power fuse is needed to protect the control circuitry from excessive power surges and protect the power source itself from a control system fault which may cause improper connection between the hot and common leads.